MSH-FThe Main Spring Housing - Fitable is intended for the higher-end custom gun where the MSH fit matters -- in other words, when it is not acceptable to have it move around and have wide and/or uneven gaps in the fit. It is made to the very high side of observed dimensions taken from a large quantity and wide cross section of 1911's and 1911 prints. It also has a feature I first did in the late '80's: a forward step for a superior magwell job that does not create a feather-edge of steel from the mag chute's rear wall. This requires milling the rear wall of the pistol's magazine chute -- or, this area of the MSH-F can simply be milled off to standard configuration.
Widths are a full .545 and .634, concentric. I've seen some wildly varied frames in terms of these dimensions and many where the frame slots are well off-center from the main body width, so all that needs to be checked in the host pistol. Rails are a full .085 wide, so it can be made a no-slop fit in any frame out there. Not that it matters that much but on a gun that's costing somewhere between $3000 and "the sky's the limit", nobody wants to feel the MSH shift in dry firing. There is not a radius or pattern on the back side; these are not for the hobbyist and I figure a 'smith that's going to cut a pattern on the MSH probably has a setup to machine the radius and may want more, or less, girth, or an angle, or what-have-you. And I have left metal on it in case anyone wants to do my style of girth reduction. The reason for the "hips" is explained in this thread: http://forum.ltwguns.com/viewtopic.php?f=6&t=9678 ... and here's a picture of "hips".
If the hips aren't needed, they are simply milled off in the fitting process which will need to take place anyway. And in the event of fitting the MSH to a frame where .545 and .085 are not quite enough, this area gives you the metal you would need to fit to any possible slot depth and width scenario.
Another feature of the MSH-F is the retaining pin hole. These are typically "all over the place" in terms of size and relative locations between frame and MSH. Some have very large, "fits anything" holes making a good fit difficult without welding or other rework. The MSH-F has a properly sized hole, properly located but to the low side of the location range, that is, a tad toward the bottom. Given that the MSH-F will be stopped upward by the forward step, and fore-aft by the rail, all the MSH retaining pin has to do is keep it from falling out, and keep it pushed up against the forward step. If the frame and MSH-F retaining pin holes don't match well fore/aft as the 'smith is fitting the MSH-F's forward step stopping point, he can simply drill the MSH-F's retaining pin hole larger, giving fore/aft clearance and "sneaking up" on a fit where there is no up/down movement, and no fore / aft interference between the MSH-F retaining pin holes in the frame and MSH-F.
Finally, there is plenty of metal on the bottom so that after getting a perfect fit everywhere else, the 'smith can mill or surface grind the bottom to perfectly match the bottom of the pistol.
As with all of my products, MSH-F materials are selected for optimal properties in use, not ease of manufacture or low cost. That means real-deal gunmetal: pre-hardened alloys, not melted-down Chinese car bodies or soft, lead-alloyed low carbon steels. It means US-Made, chrome-moly alloy steel or US-made rifle-barrel stainless. They will also be available in aluminum -- not the soft and gummy stuff but 7075 T6 bar stock.